1. Field of the Invention
The present invention relates to fabrics used in papermaking machines, and, more particularly, to fabrics used to carry fiber or, more particularly, paper webs through a drying press.
2. Description of the Related Art
For many years attempts have been made to use external air pressure to force water out of a paper web. Rather than compress a sheet at a press nip to the point where hydraulic pressure drives water out, as is the case in normal wet pressing, it was reasoned that more water could be removed, and sheet bulk could be maintained, if air pressure could be applied to supplement roller nip generated hydraulic pressures. One such attempt involves providing a multi-roller or other structure forming an air press having a closed chamber, wherein air is circulated through the chamber to convect moisture out of the paper web. Such air presses typically carry the paper web sandwiched between an upper pressing fabric and a lower anti-rewet layer.
Much attention has been given to the design of the pressing fabric and its characteristics. The construction of the pressing fabric has been thought to be the most important of the above-mentioned fabrics since it controls mechanical pressure on the paper web and the air flow therethrough. However, experimentation has shown the importance of the underneath anti-rewet layer. It has been found that rewet can have a profound effect on sheet solids after pressing. Specifically, the quality of the paper web has been found to decrease with increasing rewet. Sheet rewet can be controlled by the design of the anti-rewet layer.
What is needed in the art is an anti-rewet layer for use in air presses which can effectively minimize the amount of rewet which occurs in a fiber web during and after pressing thereof in a drying press.
The present invention provides an anti-rewet fabric or felt that includes at least one air distribution layer laminated or otherwise attached to a perforated film layer, the anti-rewet fabric having a low enough permeability so that water cannot be attracted back into a fiber web carried thereby through an air press.
The invention comprises, in one form thereof, an anti-rewet felt for carrying a fiber web through an air press. The anti-rewet felt includes at least one air distribution layer, one air distribution layer being configured for contacting the fiber web, and a perforated film layer, the perforated film layer being made of a polyester film. The perforated film layer has a first film side and a second film side, the first film side being one of laminated and attached to the one air distribution layer.
In another form thereof, the invention comprises a papermaking machine for making a fiber web. The papermaking machine includes a plurality of conveyor rolls for carrying the fiber web and first and second opposing press elements. The first press element and the second press element together form a nip therebetween. The papermaking machine further includes at least a first anti-rewet layer configured for carrying the fiber web through the nip. The first anti-rewet felt includes at least one air distribution fabric layer, one air distribution fabric layer being configured for contacting the fiber web, and a perforated film layer, the perforated film layer being made of a polyester film. The perforated film layer has a first film side and a second film side, the first film side being one of laminated and attached to the one air distribution fabric layer, the second film side being directed toward one press element.
In another form thereof, the invention comprises a method of conveying a fiber web into an air press, the air press having a nip. The method includes the step of providing an anti-rewet felt for carrying the fiber web through an air press. The anti-rewet felt includes at least one air distribution fabric layer configured for contacting the fiber web and a perforated film layer, the perforated film layer being made of a polyester film. The perforated film layer has a first film side and a second film side, the first film side being one of laminated and attached to one the air distribution fabric layer. The method further includes the step of carrying the fiber web on one air distribution fabric layer of the anti-rewet felt into the air press through the nip.
An advantage of the present invention is rewet of the fiber web after water has been removed therefrom can be greatly minimized.
A further advantage is that the perforated film layer of the anti-rewet felt or fabric increases the average air flow path length through the fabric.